Differential amplifier and data driver employing the differential amplifier

ABSTRACT

Disclosed is a differential amplifier which includes first and second input terminals, an output terminal, first and second differential pairs, and first and second current sources for supplying currents to the first and second differential pairs. The first differential pair has first and second inputs of an input pair connected to the first input terminal and the output terminal, respectively. The second differential pair has first and second inputs of an input pair connected to the second input terminal the output terminal, respectively. The differential amplifier further includes a load circuit connected to output pairs of the first and second differential pairs for outputting a signal obtained on combining outputs of the first and second differential pairs from at least one of a pair of connection nodes between the output pairs of the first and second differential pairs and the load circuit, an amplifier stage supplied with at least one signal at a connection node of the output pairs of the first and second differential pairs and the load circuit to output a voltage at the output terminal, and a current control circuit controlling the first and second current sources for controlling the ratio of currents supplied to the first and second differential pairs.

FIELD OF THE INVENTION

This invention relates to a differential amplifier, a display apparatusemploying the differential amplifier.

BACKGROUND OF THE INVENTION

Recently, a liquid crystal display (LCD) apparatus, featuring thinthickness, small weight and low power usage, has come into widespreaduse, as display apparatus, and is used predominantly in a displaysection of a mobile device, such as mobile phone or cellular phone, PDA(Personal Digital Assistant) or a notebook PC. However, of these days,the technique for forming a liquid crystal display to a large size orfor coping with moving pictures has been improved such that realizationof a large size display apparatus of a mobile type but also of a desktoptype, or a large size liquid crystal television receiver, has becomefeasible. For liquid crystal display apparatus, a liquid crystal displayapparatus of the active matrix driving system, capable of highdefinition demonstration, is currently in use. Referring first to FIG.15, a typical structure of the liquid crystal display apparatus of theactive matrix driving system is schematically described. Meanwhile, FIG.15 schematically shows, by an equivalent circuit, an essential structurecentered about the connection portion to a single pixel in a liquidcrystal display section.

In general, a display 960 of a liquid crystal display apparatus of theactive matrix driving system comprises a semiconductor substrate whichincludes a matrix array of transparent pixel electrodes 964 and thinfilm transistors (TFTs) 963 (an array of 1280 columns each composed ofthree pixels and 1024 pixel rows in the case of a color SXGA panel), aopposing substrate, in which a transparent electrode 966 is providedover its entire surface, and a liquid crystal material which is enclosedin a space defined between the semiconductor substrate and the opposingsubstrate which are mounted facing each other.

The TFTs, having the switching function, are controlled by scanningsignals. When the TFTs are turned on, the gray scale voltage,corresponding to a picture signal, is applied to pixel electrode 964.The liquid crystal is changed in transmittance by the potentialdifference between the pixel electrode 964 and the opposing electrode966. The potential difference is retained for a predetermined time by aliquid crystal capacitance 965 to display a picture.

On the semiconductor substrate, data lines 962 for delivering plurallevel voltages (gray scale voltage) to be applied to the respectivepixel electrodes 964 and scanning lines 961 for sending scanning signalsare arrayed in a lattice configuration. The numbers of the data linesand the scanning lines are 1280×3 and 1024, respectively, for the caseof the above color SXGA panel. The scanning lines 961 and the data lines962 represent large capacitive load due e.g. to the capacitancegenerated in the intersections and to the liquid crystal capacitancesandwiched between the two facing substrates.

Meanwhile, the scanning signal is supplied by a gate driver 970 to thescanning line 961, while the gray scale voltage is supplied from a datadriver 980 to the pixel electrodes 964 over a data line 962.

Data is written from one frame to another within one frame period ( 1/60sec), that is, data is selected from one pixel row to another, viz. fromone scanning line to another, and the gray scale voltage is suppliedfrom each data line during the select period.

While it is only necessary for the gate driver 970 to supply at leastbinary scanning signal, it is necessary for the data driver 980 to drivethe data lines with the gray scale voltage of multiple levelscorresponding to the number of gray levels. For this reason, adifferential amplifier capable of outputting a high precision voltage isused as a buffer unit of the data driver 980.

Recently, with the tendency towards high picture quality (i.e. towardsmultiple colors), there is an increasing demand for at least 260000colors (picture data of 6 bits for each of RGB) and for as many as26800000 colors (picture data of 8 bits for each of RGB).

Hence, the data driver, outputting the gray scale voltage relating tomulti-bit picture data, is required to output the voltages to extremelyhigh accuracy. Moreover, the number of devices of the circuitryprocessing the picture data is increased, thus leading to an increasedchip area of the data driver LSI and to increased cost. This problemwill now be discussed in detail.

FIG. 16 is a diagram illustrating the configuration of the data driver980 of FIG. 15 and, more precisely, shows a block diagram showingessential parts of the data driver 980. Referring to FIG. 16, the datadriver 980 comprises a latch address selector 981, a latch 982, a grayscale voltage generating circuit 983, a plural number of decoders 984,and a plural number of buffer circuits 985.

The latch address selector 981 determines the data latch timing, basedon a clock signal CLK. At a timing as determined by the latch addressselector 981, the latch 982 latches digital picture data and, responsiveto an STB signal (strobe signal), unanimously outputs the latched datato the respective decoders 984. The gray scale voltage generatingcircuit 983 generates a number of gray scale voltages related to thepicture data. The decoder 984 selects and outputs one of the gray scalevoltages corresponding to the input data. The buffer circuit 985 issupplied with the gray scale voltage output from the decoder 984 toamplify the current to issue an output voltage Vout.

For example, in case of 6-bit picture data being input, the number ofgray levels is 64, with the gray scale voltage generating circuit 983generating 64-level gray scale voltages. The decoder 984 includes acircuit for selecting one gray scale voltage from the 64 level grayscale voltages.

For example, in case of 8-bit picture data being input, the number ofgray levels is 256, with the gray scale voltage generating circuit 983generating 256-level gray scale voltages. The decoder 984 includes acircuit for selecting one gray scale voltage from the 256 level grayscale voltages.

With increase in the number of bits, the gray scale voltage generatingcircuit 983 or the decoder 984 is increased in circuit scale. Forexample if the number of bits is increased from 6 to 8, the circuitscale is increased by a factor not less than four. Thus, with increasein the number of bits, the chip area of the data driver LSI is increasedto raise the cost.

There is described in Patent Document 1, specified below, a structurefor suppressing the chip area of the data driver LSI from increasing,despite increase in the number of bits. An illustrative structure asproposed in the Patent Document 1 is shown in FIG. 17, corresponding toFIG. 16 of the Patent Document 1.

Referring to FIG. 17, this data driver differs from the data drivershown in FIG. 16 as to the structure of a gray scale voltage generatingcircuit 986, a decoder 987 and a buffer circuit 988. In the data driver,shown in FIG. 17, the gray scale voltage generating circuit 986generates the gray scale voltage, every two gray levels, to decrease thenumber of gray level voltage lines of the decoder 987 to approximatelyone-half that of the decoder 984 of FIG. 16. The decoder 987 selects twogray level voltages, depending on picture data, to send out the selectedvoltages to the buffer circuit 988, which buffer circuit then is able toapply current amplification to the two gray level voltages and a graylevel voltage intermediate between these two gradation voltages tooutput the resulting voltage.

In the configuration disclosed in the Patent Document 1, provided withthe buffer circuit 988, supplied with two gray scale voltages toselectively output one of the two gray scale voltages and the voltageintermediate between the two gradation voltages, it is contemplated toreduce the number of the gray scale voltage lines of the decoder 987 andhence the circuit scale of the decoder 987 to save the floor and hencethe production cost.

In Patent Documents 2 and 3, as specified below, there is described astructure for significantly saving the data recording area of a circuitfor converting multi-bit digital signals into analog signals(digital-to-analog converter, abbreviated to DAC), as an interpolationDAC. The technique disclosed in Patent Document 3 is an improvement overthat of the Patent Document 2 and is relevant to the configuration forimproving the performance of output voltage accuracy. In the following,the Patent Document 3 is described. Referring to FIG. 18, this DAC ismade up by a resistor string, outputting n analog voltages from itsrespective taps, a first group of switches, made up of n switches offrom S1 a to Sna, for selecting one voltage V1 from each tap, a secondgroup of switches, made up of n switches of from S1 b to Snb, forselecting a voltage V2, neighboring to this voltage V1, a third group ofswitches, made up of switches SW1, SW2 and SW3, for selecting one of V1and V2, and an amplifier 200, made up of four differential pairs (Q0A,Q0B), (Q1A, Q1B), (Q2A, Q2B), and (Q3A, Q3B) which are driven bydistinct current sources. The output pairs of the four differentialpairs are connected in common to input/output pairs of the currentmirror circuit (QL1, QL2), while output signals of the four differentialpairs are differentially output to a differential amplifier 205, whichissues an output voltage Vout at its output terminal. One of inputs ofinput pairs of the four differential pairs (Q0A, Q0B), (Q1A, Q1B), (Q2A,Q2B), and (Q3A, Q3B) is connected to an output terminal in a feedbackconfiguration. As for the other inputs of the input pairs of the fourdifferential pairs (Q0A, Q0B), (Q1A, Q1B), (Q2A, Q2B), and (Q3A, Q3B),one is connected to the first group of the switches, selecting thevoltage V1, with the remaining three being connected to the third groupof switches SW1, SW2 and SW3 adapted for selecting one of V1 and V2.Turning to the operation of the DAC, the K'th switches (Ska and Skb) ofthe first and second group of switches (S1 a . . . Sna), (S1 b . . .Snb) are turned on by an output of an MSB (Most Significant Bit) subworddecoder, based on upper order bit signals of the input data, to selectvoltages of neighboring taps as V1 and V2, and further the switching ofthe switches of the third group of switches (SW1, SW2, SW3) iscontrolled by an output of an LSB (Least Significant Bit) subworddecoder, based on lower bit signals of the input data.

By the selecting conditions of the third group of switches (SW1, SW2,SW3), four level voltages Vo1 to Vo4, corresponding to interior divisionof the voltages V1 and V2, shown in FIG. 19 at different interiordivision ratios, are output. Specifically,

If the three switches SW1, SW2, SW3 of the third group of switches allselect the voltage V1, Vo1 equal to the voltage V1 is output.

If two of the three switches SW1, SW2, SW3 select the voltage V1 and theremaining one selects the voltage V2, Vo2 is output.

If one of the three switches selects the voltage V1 and the remainingtwo select the voltage V2, Vo3 is output.

If all of the three switches select the voltage V2, Vo4 is output.

If the four level voltages Vo1 to Vo4 are to be output linearly to ahigh voltage accuracy, it is necessary that the four differential pairs(Q0A, Q0B), (Q1A, Q1B), (Q2A, Q2B), and (Q3A, Q3B) are made up oftransistors of the same size, and that the current of the currentsources for driving the differential pairs, is controlled to a constantvalue. By this configuration and switch control, described above, theDAC of FIG. 18 is able to output a sum total of 4n level voltages by theMSB and LSB subwords.

By applying the principle of this DAC to the gray scale voltagegenerating circuit 983, decoders 984 and to the buffer circuits 985 ofFIG. 16, it becomes possible to reduce significantly the number of grayscale voltages output from the gray scale voltage generating circuit983, to reduce the number of devices for selecting the gray scalevoltage and to reduce the circuit scale significantly.

[Patent Document 1]

Japanese Patent Kokai Publication No. JP-P2001-34234 (FIG. 16)

[Patent Document 2]

U.S. Pat. No. 5,396,245 (FIG. 5)

[Patent Document 3]

U.S. Pat. No. 6,246,351 (FIG. 2)

SUMMARY OF THE DISCLOSURE

Meanwhile, with the DAC shown in FIG. 18, the two voltages V1 and V2 canbe divided by different interior division ratios, by the amplifier 200,having four differential pairs, to output four levels. The number oflevels, excluding the two voltages V1 and V2, is three.

That is, if, in the principle of the DAC, shown in FIG. 18, a levelsexcluding the two voltages V1 and V2 are to be output, it is necessaryto provide (a+1) differential pairs and current sources driving them.Hence, the larger the number of levels interiorly dividing the twovoltages V1 and V2, the larger becomes the number of amplifier devices200 and the larger becomes the circuit size.

Accordingly, it is therefore an object of the present invention toprovide an area saving differential amplifier with which it is possibleto increase the number of levels interiorly dividing the two voltageswithout increasing the number of the differential pairs.

It is a further object of the present invention to provide a data driverof a display apparatus in which the circuit scale of the gray levelvoltage generating circuit, decoder and the amplifier may significantlybe reduced by employing the aforementioned differential amplifier.

It is yet another object of the present invention to provide a displayapparatus of low production cost through use of the above data driver.

The above and other objects are attained by a differential amplifier inaccordance with one aspect of the present invention, which comprises

first and second input terminals;

an output terminal;

first and second differential pairs;

first and second current sources for supplying currents to said firstand second differential pairs, respectively;

said first differential pair having first and second inputs constitutingan input pair thereof connected to said first input terminal and saidoutput terminal, respectively;

said second differential pair having first and second inputsconstituting an input pair thereof connected to said second inputterminal and said output terminal, respectively;

a load circuit connected to output pairs of said first and seconddifferential pairs, for outputting a signal obtained on combiningoutputs of said first and second differential pairs from at least one ofrespective connection node between the output pairs of said first andsecond differential pairs and said load circuit;

an amplifier stage for receiving at least one signal at a connectionnode between said output pairs of said first and second differentialpairs and said load circuit and for outputting a voltage at said outputterminal; and

a current control circuit for controlling said first and second currentsources to control the ratio of currents supplied to said first andsecond differential pairs, respectively.

Preferably, in the differential amplifier according to the presentinvention, the load circuit is formed by a current mirror circuit.

Preferably, the differential amplifier according to the presentinvention comprises: first and second voltage supply terminals forreceiving preset voltages; and an input control circuit for controllingthe connection between said first and second voltage supply terminalsand said first and second input terminals.

Preferably, in the differential amplifier according to the presentinvention, the first and second current sources each include atransistor. The current control circuit selects a bias voltage, out of aplurality of bias voltages supplied, based on an input control signal,and sends the selected bias voltage to control terminals of thetransistors constituting the first and second current sources to controlthe ratio of the currents to be supplied to the first and seconddifferential pairs.

Preferably, in the differential amplifier according to the presentinvention, the current control circuit controls the first and secondcurrent sources so that the sum of the currents supplied to the firstand second differential pairs will be constant.

Preferably, in the differential amplifier according to the presentinvention, said first and second differential pairs are formed so thatthe transistors of the same pair are of the same transistorcharacteristic.

Preferably, in the differential amplifier according to the presentinvention, the transistor pairs of said first and second differentialpairs are of the same size, the ratio of the currents through said firstand second differential pairs is adjusted, and a voltage correspondingto one of levels which divide the potential difference between saidfirst and second input terminals by three is output from said outputterminal.

Preferably, in the differential amplifier according to the presentinvention, the currents of said first and second current sources differfrom each other; and first and second voltages are applied to said firstand second input terminals, respectively.

A first voltage level, corresponding to said first voltage, is outputfrom said output terminal, under interruption of the current of saidsecond current source,

a second voltage level, corresponding to one of voltage levels whichdivide the potential difference between said first and second voltagesby three, is output from said output terminal, with the current of saidfirst current source being larger than the current of said secondcurrent source,

a third voltage level, corresponding to the other of voltage levelswhich divide the potential difference between said first and secondvoltages by three, is output from said output terminal, with the currentof said second current source being larger than the current of saidfirst current source, and

a fourth voltage level, corresponding to said second voltage, is outputfrom said output terminal, under interruption of the current of saidfirst current source.

A differential amplifier in accordance with another aspect of thepresent invention comprises:

first and second input terminals;

an output terminal;

first and second differential pairs of the first conductivity typehaving first inputs constituting respective input pairs thereofconnected to said first and second input terminals, respectively andhaving second inputs constituting respective the input pairs thereofconnected to said output terminal;

first and second current sources for supplying the current to said firstand second differential pairs, respectively;

a first load circuit connected to output pairs of said first and seconddifferential pairs;

third and fourth differential pairs of the second conductivity type,having first inputs constituting respective input pairs thereofconnected to said first and second input terminals, respectively andhaving second inputs constituting the respective input pairs thereofconnected to said output terminal;

third and fourth current sources supplying the current to said third andfourth differential pairs, respectively;

a second load circuit connected to output pairs of said third and fourthdifferential pairs;

at least one output of an output pair of said first differential pairand one output of an output pair of said second differential pair beingconnected in common;

a first amplifier stage having an input terminal connected to a commonconnection node between one output of an output pair of the firstdifferential pair, and one output of an output pair of the seconddifferential pair and having an output terminal connected to said outputterminal;

at least one output of an output pair of said third differential pairand one output of an output pair of said fourth differential pair beingconnected in common;

a second amplifier stage having an input terminal connected to a commonconnection node between one output of an output pair of the thirddifferential pair and one output of an output pair of the fourthdifferential pair and having an output terminal connected to said outputterminal;

a first current control circuit for controlling said first and secondcurrent sources to control the ratio of currents supplied to said firstand second differential pairs; and

a second current control circuit for controlling said third and fourthcurrent sources to control the ratio of currents supplied to said thirdand fourth differential pairs.

Preferably, in the differential amplifier according to the presentinvention, each of said first to fourth current sources includes atransistor; said first current control circuit is supplied with aplurality of bias voltages and selects a bias voltage based on an inputcontrol signal to supply the selected bias voltage to control terminalsof transistors constituting said first and second current sources tocontrol the current ratio of the currents supplied to said first andsecond current sources;

said second current control circuit is supplied with a plurality of biasvoltages and selects a bias voltage based on an input control signal tosupply the selected bias voltage to control terminals of transistorsconstituting said third and fourth current sources to control thecurrent ratio of the currents supplied to said third and fourth currentsources.

A differential amplifier in accordance with a further aspect of thepresent invention, comprises

first and second input terminals;

an output terminal;

first and second differential pairs having at least two transistorcharacteristics different from each other; and

first and second current sources for supplying the currents to saidfirst and second differential pairs, respectively;

said first differential pair having first and second inputs constitutingan input pair thereof connected to said first input terminal and saidoutput terminal, respectively;

said second differential pair having first and second inputsconstituting an input pair thereof connected to said second inputterminal and said output terminal;

a load circuit connected to output pairs of said first and seconddifferential pairs, for outputting a signal obtained on combiningoutputs of said first and second differential pairs from at least one ofa pair of connection nodes between said output pairs of said first andsecond differential pairs and said load circuit; and

an amplifier stage supplied with at least one signal at a connectionnode between said output pairs of said first and second differentialpairs and said load circuit to output a voltage to said output terminal.

Preferably, the differential amplifier according to the presentinvention, may further comprise two voltage supply terminals forreceiving first and second voltages, respectively, and an input controlcircuit for controlling the connection or non-connection of the twovoltage supply terminals and the first and second input terminals.

Preferably, in the differential amplifier according to the presentinvention, the characteristics of the transistor pairs of said first andsecond differential pairs are set to be different each other, and theconnection between the two voltage supply terminals and the first andsecond input terminals being controlled by the input control circuit,and a voltage corresponding to one of voltage levels which divide thepotential difference between said first and second input terminals isoutput from said output terminal by three.

Preferably, in the differential amplifier according to the presentinvention, the W/L ratio (ratio of the channel width W to the channellength L) of the transistor pairs of said first differential pairs arelarger than that of said second differential pairs, and the first andsecond voltages are selectively applied to said first and second inputterminals.

A first voltage level, corresponding to said first voltage is outputfrom said output terminal, with said first and second input terminalsboth being at a first voltage,

a second voltage level corresponding to one of voltage levels whichdivide the potential difference between said first and second voltagesby three, is output from said output terminal, with said first andsecond input terminals being at first and second voltages, respectively,

a third voltage level corresponding to one of voltage levels whichdivide the potential difference between said first and second voltagesby three, is output from said output terminal, with said first andsecond input terminals being at second and first voltages, respectively,and

a fourth voltage level corresponding to said second level is output fromsaid output terminal, with said first and second input terminals bothbeing at said second voltage.

A differential amplifier in accordance with further aspect of thepresent invention comprises

first and second input terminals;

an output terminal;

first and second differential pairs;

first and second current sources for supplying the current to said firstand second differential pairs;

said first differential pair having first and second inputs constitutingan input pair thereof connected to said first input terminal and saidoutput terminal, respectively;

said second differential pair having first and second inputsconstituting an input pair thereof connected to said second inputterminal and said output terminal, respectively;

a load circuit connected to output pairs of said first and seconddifferential pairs, for outputting a signal obtained on combiningoutputs of said first and second differential pairs from at least one ofa pair of connection nodes between said output pairs of said first andsecond differential pairs and said load circuit;

an amplifier stage supplied with at least one signal at connection nodesof said output pairs of said first and second differential pairs andsaid load circuit to output a voltage to said output terminal; and

a selection circuit supplied with a plurality of voltages different fromone another, for selecting, from among said voltages different from oneanother, two voltages, including the same voltage, based on a selectioncontrol signal, to supply the selected two voltages to said first andsecond input terminals;

one output voltage being output for a combination of said two voltagesselected by said selection circuit.

Preferably, in the differential amplifier according to the presentinvention, said selection circuit is supplied with first to fourthvoltages (A to D) having voltage values different from one another andsends any of paired voltages:

(1) first and first voltages (A, A),

(2) first and second voltages (A, B),

(3) second and second voltages (B, B),

(4) first and third voltages (A, C),

(5) second and third voltages (B, C) or first and fourth voltages (A, D)

(6) second and fourth voltages (B, D),

(7) third and third voltages (C, C),

(8) third and fourth voltages (C, D) and

(9) fourth and fourth voltages (D, D)

to said first and second input terminals.

said differential amplifier is able to output nine at the maximum ofvoltages different from one another from said output terminal.

Preferably, in the differential amplifier according to the presentinvention, the first, third, seventh and ninth voltage levels from amongnine voltage levels, are supplied as said first to fourth voltagelevels.

Preferably, in the differential amplifier according to the presentinvention, said selection circuit is supplied with first to sixthvoltages (A to F) having voltage values different from one another, andsends any of paired voltages:

(1) first and first voltages (A, A),

(2) first and second voltages (A, B),

(3) second and second voltages (B, B),

(4) first and third voltages (A, C),

(5) second and third voltages (B, C)

(6) first and fourth voltages (A, D),

(7) third and third voltages (C, C) or second and fourth voltages (B, D)

(8) first and fifth voltages (A, E),

(9) third and fourth voltages (C, D) or second and fifth voltages (B, E)or first and sixth voltages (A, F),

(10) second and sixth voltages-(B, F)

(11) fourth and fourth voltages (D, D) or third and fifth voltages (C,E)

(12) third and sixth voltages (C, F)

(13) fourth and fifth voltages (D, E)

(14) fourth and sixth voltages (D, F)

(15) fifth and fifth voltages (E, E)

(16) fifth and sixth voltages (E, F)

(17) sixth and sixth voltages (F, F)

to said first and second input terminals.

said differential amplifier is able to output 17 at the maximum ofvoltages different from one another from said output terminal.

Preferably, in the differential amplifier according to the presentinvention, first, third, seventh, eleventh, fifteenth and seventeenthvoltage levels from among 17 voltage levels are supplied as said firstto sixth voltages.

Preferably, in the differential amplifier according to the presentinvention, when m voltages, different from one another, m being amultiple of 8 plus 1, are output, the number of voltages, supplied asinput voltages, is [{(m−1)/4}+2]. Two voltages, inclusive of overlappingvalues (the same voltage value), may be selected from among the[{(m−1)/4}+2] voltages, and the so selected two voltages are sent to thefirst and second terminals. The differential amplifier is able to outputm voltage levels at the output terminal. For the first to m'th voltagelevels output, the voltages supplied as input voltages may includefirst, (3+4×k)'th and the m'th voltage levels, k being an integer of 0,1, . . . , (m−5)/4, against the first to m'th output voltage levels.

Preferably, in the differential amplifier according to the presentinvention, said amplifier stage includes a transistor connected betweena power supply and said output terminal and adapted forcharging/discharging said output terminal.

Preferably, in the differential amplifier according to the presentinvention, at least one of said first and second amplifier stagesincludes a transistor connected between a power supply and said outputterminal and adapted for charging/discharging said output terminal.

A digital-to-analog converter according another aspect of the presentinvention comprises:

a resistor string connected between said first voltage supply terminaland a second voltage supply terminal and outputting a plurality ofvoltage levels different from one another from respective taps thereof;and

a switching circuit for selectively outputting two voltages, based oninput data, from among a plurality of voltage levels, different from oneanother, output from respective taps of said resistor string; and

the differential amplifier according to the present invention, as anamplifier circuit supplied with the two voltages output from said switchcircuit and outputting a voltage relating to said input data.

A digital-to-analog converter according another aspect of the presentinvention comprises:

a resistor string connected between a first voltage supply terminal anda second voltage supply terminal and outputting a plurality of voltagelevels different from one another from respective taps thereof; and

a switching circuit for selectively outputting two voltages, inclusiveof the same voltages, based on input data, from among a plurality ofvoltage levels, different from one another, output from respective tapsof said resistor string; and

the differential amplifier according to the present invention, as anamplifier circuit supplied with the two voltages output from said switchcircuit and outputting a voltage relating to said input data.

In another aspect, the present invention provides a data driver for adisplay apparatus comprising a gray level voltage generating circuitgenerating a plurality of voltage levels, a decoder outputting at leasttwo voltages selected from the plural voltages, based on input data, anda differential amplifier, described above, as an amplifier circuitsupplied with the voltage output from the decoder and outputting avoltage relating to the input data at an output terminal.

In yet another aspect, the present invention provides a controllingmethod for enabling m voltage levels to be output, m being such thatm=8×1+1. 1 being a positive integer, using a differential amplifier,said differential amplifier including at least two differential pairs,each having a non-inverting input terminal supplied with a signalvoltage and having an inverting input terminal connected in common to anoutput terminal, each non-inverting input terminal being supplied withfirst and second voltages, inclusive of the same voltage values, saiddifferential amplifier outputting said first and second voltages and avoltage which is a division of said first and second voltages; saidmethod comprising: the steps of

generating at least [{(m−1)/4+2}] voltage levels, composed of first,3+4×k'th, k being an integer of 0, 1, . . . , (m−5)/4, and m'th voltagelevels, for the first to the m'th voltage levels;

selecting two voltages, inclusive of the voltage values, from among[{(m−1)/4+2}] voltages supplied as input voltages; and

supplying the two voltages selected to at least said non-inverting inputterminal of said two differential pairs.

The meritorious effects of the present invention are summarized asfollows.

The present invention gives rise to a meritorious effect that, in adifferential amplifier having two differentia pairs of the same polaritytype, two input voltages and two or more level voltages, interiorlydividing the interval of the two input voltages, may be outputresponsive to the two input voltages.

The present invention also gives rise to a meritorious effect that adifferential amplifier capable of outputting three voltage levels,namely two input voltages and an intermediate voltage, responsive to thetwo input voltages, may be realized, in which, by supplying, as twoinput voltages, two preset combinatorial voltages, including the samevoltage, from among n voltages, n being a positive natural number notless than 3, it is possible to output substantially equally spaced apart(2n+1) or more level voltages.

The present invention also gives rise to a meritorious effect that, byemploying, in an output buffer unit of a data driver of a displayapparatus, a differential amplifier having two differentia pairs of thesame polarity type, in which two input voltages and two or more levelvoltages, interiorly dividing the interval of the two input voltages,may be output responsive to the two input voltages, it is possible toreduce the number of gray level voltages generated in the gray levelvoltage generating circuit to reduce the circuit scale of the decoderand the output buffer unit to save the data drive area and hence theproduction cost.

The present invention also gives rise to a meritorious effect that, byemploying, in an output buffer unit of a data driver of a displayapparatus, a differential amplifier capable of outputting three voltagelevels, namely two input voltages and an intermediate voltage,responsive to the two input voltages, in which, by supplying, as twoinput voltages, two preset combinatorial voltages, including the samevoltage, from among n voltages, n being a positive natural number notless than 3, substantially equally spaced apart (2n+1) or more levelvoltages may be output, it is possible to reduce the number of graylevel voltages generated in the gray level voltage generating circuit toreduce the circuit scale of the decoder and the output buffer unit tosave the data driver area and hence the production cost.

Moreover, the present invention also gives rise to a meritorious effectthat, by employing the data driver, reduced in area, for a displayapparatus, the display apparatus may be lowered in cost and the framingarea of the display apparatus may also be decreased.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description in conjunction with the accompanying drawingswherein only the preferred embodiments of the invention are shown anddescribed, simply by way of illustration of the best mode contemplatedof carrying out this invention. As will be realized, the invention iscapable of other and different embodiments, and its several details arecapable of modifications in various obvious respects, all withoutdeparting from the invention. Accordingly, the drawing and descriptionare to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the configuration of a differentialamplifier according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating the configuration of a differentialamplifier according to a second embodiment of the present invention.

FIG. 3 is a graph of V-I characteristics illustrating the firstoperation of the first embodiment of the present invention.

FIG. 4 is a graph of V-I characteristics illustrating the secondoperation of the first embodiment of the present invention.

FIG. 5 is a graph of V-I characteristics illustrating the firstoperation of the second embodiment of the present invention.

FIG. 6 is a diagram illustrating the configuration of adigital-to-analog converter according to a third embodiment of thepresent invention.

FIG. 7 is a diagram illustrating the configuration of adigital-to-analog converter according to a fourth embodiment of thepresent invention.

FIG. 8 shows an exemplary method for selecting a bi-level input voltageaccording to the fourth embodiment of the present invention.

FIG. 9 shows another exemplary method for selecting a bi-level inputvoltage according to the fourth embodiment of the present invention.

FIG. 10 is a diagram illustrating the configuration of a differentialamplifier of the present invention used for simulation.

FIG. 11 is a graph illustrating input/output characteristics in a thirdoperation of the differential amplifier of the present invention.

FIG. 12 a graph illustrating input/output characteristics in a fourthoperation of the differential amplifier of the present invention.

FIG. 13 a graph illustrating further input/output characteristics in thefourth operation of the differential amplifier of the present invention.

FIG. 14 a graph illustrating input/output characteristics in a fifthoperation of the differential amplifier of the present invention.

FIG. 15 is a diagram illustrating the configuration of a displayapparatus.

FIG. 16 is a diagram illustrating the configuration of a conventionaldata driver.

FIG. 17 is a diagram illustrating a modification of the configuration ofthe conventional data driver.

FIG. 18 is a diagram illustrating the configuration of a conventionaldigital-to-analog converter.

FIG. 19 illustrates how the input/output level in FIG. 19 is coped with.

FIG. 20 is a diagram illustrating the configuration of a differentialamplifier according to a fifth embodiment of the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be described in more detail with reference tothe drawings. FIG. 1 is a diagram illustrating the configuration of adifferential amplifier according to a first embodiment of the presentinvention. Referring to FIG. 1, the differential amplifier according tothe first embodiment of the present invention includes a firstdifferential pair (101, 102), a second differential pair (103, 104), acurrent mirror circuit (111, 112), which is connected to output pairs ofthe first and second differential pairs and composes a common activeload for the first and second differential pairs, an amplifier circuit6, which is supplied with an output signal of the current mirror circuit(111, 112) and outputs a voltage Vout at an output terminal 3, and acurrent control circuit 7 which is for controlling respective currents(I1, I2) which flow through the first and second differential pairs. Itis noted that the differential pair, which is constituted by a pair oftransistors (101, 102), is referred to below as a differential pair(101, 102).

The transistor 101 of the first differential pair has a control terminal(gate) connected to an input terminal T1, and the transistor 103 of thesecond differential pair has a control terminal (gate) connected to aninput terminal T2. The control terminals of the transistors 102 and 104of the first and second differential pairs are connected to an outputterminal 3, with an output voltage Vout being fed back to an input side.The terminal voltages of the input terminals T1 and T2 are designated asVT1 and VT2, respectively.

The current control circuit 7 comprises a current control transistor126, which is connected to a low potential power supply VSS and isadapted for driving the current I1 flowing in the first differentialpair (101, 102), a current control transistor 127, which connected tothe low potential power supply VSS and is adapted for driving thecurrent I2 flowing in the second differential pair (103, 104), and agroup of switches (SW1 a to SW4 a, SW1 b to SW4 b), which are capable ofselectively supplying plural voltages (VB1 to VB4) to respective controlterminals (gates) of the current controlling transistors 126 and 127.

FIG. 1 shows a typical configuration of a current mirror circuit (111,112) which is connected between respective output pairs of the firstdifferential pair (101, 102) and the second differential pair (103, 104)and a high potential power supply VDD. Specifically, the current mirrorcircuit (111, 112) includes transistors 111 and 112. A source of thetransistor 112 is connected to the high potential power supply VDD, anda drain and a gate of the transistor 112 are connected in diodeconnection to constitute an input terminal of the current mirrorcircuit. A source and a gate of the transistor 111 are connected to thepower supply VDD and the gate of the transistor 112, respectively and adrain of the transistor 111 constitutes an output terminal of thecurrent mirror circuit. An input terminal of the current mirror circuit(drain of the transistor 112) is connected in common to the drains ofthe transistors 102 and 104, and an output terminal of the currentmirror circuit (drain of the transistor 111) is connected in common tothe drains of the transistors 101 and 103 to constitute an outputterminal 4 of the differential stage which is connected to an inputterminal of an amplifier stage 6.

The differential amplifier, shown in FIG. 1, is able to output a voltageresulting from interior division of the voltages VT1 and VT2 at theinput terminals T1 and T2 by a preset interior division ratio. A pluralnumber of levels of voltages, with the different interior divisionratios, may be output under control by the current control circuit 7 ofthe current ratio of the currents I1 and I2 flowing through the currentcontrol transistors 126 and 127, respectively.

FIG. 2 is a diagram illustrating the configuration of a differentialamplifier according to a second embodiment of the present invention. Inthe differential amplifier of the above-described first embodiment,shown in FIG. 1, the differential input terminals T1 and T2 of thetransistors 101 and 103 are supplied with preset voltages VT1 and VT2,respectively. In the differential amplifier of the second embodiment,shown in FIG. 2, there is provided an input control circuit 8 forselectively supplying two control voltages (V1, V2) to the terminals T1and T2, as VT1 and VT2, respectively. In FIG. 2, the first differentialpair (101, 102) and the second differential pair (103, 104) may beformed by transistors having respective different characteristics.

On the other hand, the control terminals of the current controltransistors 126 and 127 are supplied with fixed voltages VB11 and VB12,respectively, such that the currents I1 and I2 are controlled to apreset current ratio. In other respects, the differential amplifier ofthe present embodiment is similar to that shown in FIG. 1. Hence, thedifferential amplifier of FIG. 2 is able to output a voltagecorresponding to interior division of the voltages VT1 and VT2 at theterminals T1 and T2 by an interior division ratio corresponding to thecurrent ratio of the currents I1 and I2.

The input control circuit 8 comprises switches 151 and 152, which areconnected between a terminal supplied with the control voltage V1 andterminals T1 and T2, respectively, and switches 154, 155, which areconnected between a terminal supplied with a control voltage V2 andterminals T1 and T2, respectively. By controlling these switches, thetwo supply voltages (V1, V2), inclusive of the overlapping values, mayselectively be supplied to the terminals T1 and T2.

In the differential amplifier, shown in the second embodiment of thepresent invention, a voltage obtained by interior division of thevoltages VT1 and VT2 at the terminals T1 and T2 with a preset interiordivision ratio may be output. Hence, by selectively supplying thevoltages V1 and V2 to the terminals T1 and T2, respectively, by theinput control circuit 8, it is possible to output voltages of the samelevels as the voltages V1 and V2 and voltages of two levelscorresponding to interior division of the voltages V1 and V2.

In the differential amplifiers of the first and second embodiments,shown in FIG. 1, the first differential pair (101, 102) and the seconddifferential pair (103, 104) are n-channel transistors (same polaritytype transistors). However, the differential amplifier including twodifferential pairs of p-channel transistors may, of course, be capable.

The operation of each of the first and second embodiments, shown inFIGS. 1 and 2, will now be described.

Initially, the operation of the above-described first embodiment of thedifferential amplifier, shown in FIG. 1, for a case where the respectivevoltages VT1 and VT2 of the terminals T1 and T2 are of different voltagevalues, and satisfy the condition of VT1<VT2, will be described withreference to FIGS. 3 and 4.

FIG. 3 illustrates the operation of the differential amplifier of FIG. 1for a case where the current ratio of the currents I1 and I2 is equal tothe unity.

It is assumed that the transistors of the two differential pairs (101,102) and (103, 104) of the differential amplifier of FIG. 1 are of thesame characteristics.

The current flowing through the transistors 101, 102, 103 and 104(drain-to-source currents) are represented by Ia, Ib, Ic and Id,respectively.

FIG. 3 shows the relationship between the drain-to-source current Idsand voltage from the voltage of the power supply VSS, plotted on theordinate and the abscissa, respectively and, more specifically, showstwo characteristic curves (Ids-Vg characteristics) of the transistors101 and 102 and the transistors 103 and 104.

In FIG. 3, the characteristic curve 1 is associated with the transistors101 and 102, while the characteristic curve 2 is associated with thetransistors 103 and 104. The differential pair of the transistors 101and 102 has operating points a, b, on its characteristic curve, whilethe differential pair of the transistors 103 and 104 has operatingpoints c, d, on its characteristic curve.

Meanwhile, since the transistors 101 to 104 exhibit the samecharacteristics, the two characteristic curves are simply shifted alongthe abscissa. The reason the two characteristic curves of the twodifferential pairs are shifted from each other is that the tails(terminals connected to sources) are able to undergo individualpotential variations.

As for the currents through the transistors 101 to 104, under the aboveconditions, the equation (1) holds in connection with the differentialpair (101, 102).Ia+Ib=I1  (1)

Also, the equation (2) holds in connection with the differential pair(103, 104).Ic+Id=I2  (2)

As for the current mirror circuit (111, 112), the equation (3) holds.Ia+Ic=Ib+Id  (3)

Moreover, from the condition of the current ratio relationship (I1=I2)and from the equations (1) and (2), the equation (4) is derived.Ia+Ib=Ic+Id  (4)

It is assumed that output terminal 4 of the current mirror circuit(drain of the transistor 111) applies only a voltage signal to theamplifier circuit 6, there flowing no steady-state current across theoutput terminal 4 of the current mirror circuit and the output terminal3 through the amplifier circuit 6.

Solving the above equations (3) and (4), the relationship of theequations (5) and (6) are derived.Ia=Id  (5)Ib=Ic  (6)

That is, under the above conditions, the state in which the currentsflowing through the transistors 101 and 104 are equal to each other(Ia=Id) and in which currents flowing through the transistors 102, 103are equal to each other (Ib=Ic) represents a stable state of thedifferential amplifier of FIG. 1.

The operating points a to d at this time are at the points ofintersection of the characteristic curves 1 and 2 and straight linesIds=Ia=Id and Ids=Ib=Ic. Since the operating points b and d of thetransistors 102 and 104 have V=Vout on the abscissa V of FIG. 3 incommon, the operating points b and d are determined, and operatingpoints a and c are also determined. This state is shown in FIG. 3. Thefigure obtained on interconnecting the four operating points on thecharacteristic curves of the transistors 101 to 104 is a parallelogram.Since the sides ad and bc of the parallelogram are equal to each other,the output voltage Vout is an intermediate voltage between the voltagesVT1 and VT2.

From the above, it follows that, when the transistors 101 to 104 are ofequal characteristics, and the ratio of currents I1 and I2 is 1:1, theoutput voltage Vout is a voltage interiorly dividing the voltages VT1and VT2 by the ratio of 1:1 (intermediate voltage).

Referring to FIG. 4, the case in which the current ratio is changed willnow be described. FIG. 4 shows the operation in which the transistors ofthe two differential pairs (101, 102) and (103, 104) of the differentialamplifier of FIG. 1 are of the same characteristics and in which thecurrents I1 and I2 are set so that I1 is approximately twice as large asI2 (I1=I2×2).

The terminal voltages VT1 and VT2 are also set so that VT1<VT2.

FIG. 4 shows, similarly to FIG. 3, the relationship between thedrain-to-source current Ids (ordinate) and the voltage from the powersupply VSS. The characteristic curves 1 and 3 stand for thecharacteristics of the transistors 101 and 102 and the transistors 103and 104, respectively, with the transistors 101 and 102, 103 and 104 ofthe two differential pairs having operating points a, b, c and d on therespective characteristic curves, respectively. In FIG. 4, the equations(1) to (3) are valid as the current relationships for the transistors101 to 104.

From the relationship of the currents I1 and I2 (I1=I2×2), the followingequation (7) is valid.Ia+Ib=2×(Ic+Id)  (7)

Solving the above equations (3) and (7), the following equations (8) and(9) are derived.Ia=(Ic+3×Id)/2  (8)andIb=(3×Ic+Id)/2  (9)

In case the current I1 of the current source 126 differs from thecurrent I2 of the current source 127, the simple equation for the caseof FIG. 3 does not hold, but the output stable state of the differentialamplifier of FIG. 1 is obtained for the case such as that of FIG. 4.

Meanwhile, the operating points b and d of the transistors 102 and 104have V=Vout on the abscissa in common.

Since the current ratio of the currents I1 and I2 is such that I1 is setto approximately twice as large as I2, the output voltage Vout assumes avalue closer to the voltage VT1. Thus, by adjusting the current ratio ofthe currents I1 and I2, it is possible to take out an output voltageVout interiorly dividing the voltages VT1 and VT2 with a ratio ofapproximately 1:2.

Meanwhile, the current ratio of the currents I1 and I2 can beapproximated to some extent to a reciprocal of the interior divisionratio of the voltages VT1 and VT2 by the output voltage Vout, subject toproper adjustment of the transistor characteristics of the twodifferential pairs or the values of the currents I1, I2, which maybecome influencing factors to a more or less extent. However, even inthis case, there is generated an increasing shift in the relationshipbetween the reciprocal of the interior division ratio and the currentratio with increase in the voltage difference between VT1 and VT2.

In the foregoing explanation, referenced to FIG. 4, it is assumed thatthe currents I1 and I2 of the current sources 126 and 127 are such thatI1>I2. It may however be readily understood that, if I2=I1×2, such anoutput voltage Vout may be taken out which interiorly divides thevoltages VT1 and VT2 with a ratio of 2:1.

In the differential amplifier of FIG. 1, the input voltages to theterminals T1 and T2 are generally fixed. If the current ratio of thecurrents I1 and I2 is such that I1>I2, the output voltage Vout becomes avoltage interiorly dividing the voltages VT1 and VT2 at a point closerto the voltage VT1 and, if the same current ratio is such that I1<I2,the output voltage becomes a voltage interiorly dividing the voltagesVT1 and VT2 at a point closer to the voltage VT2. The larger thedifference between the two currents, the output voltage may be furthercloser to the voltage VT1 or VT2. By optimizing the condition, it ispossible to correlate the current ratio of the currents I1 and I2 withthe reciprocal of the interior division ratio of the voltages VT1 andVT2. Thus, with the differential amplifier of FIG. 1, plural levelvoltages, interiorly dividing the voltages VT1 and VT2, may be output byswitching controlling of the current ratio of the currents I1 and I2 bythe current control circuit 7.

Meanwhile, if the voltages VT1 and VT2 are set to different values,however, any one of the plural voltages (VB1 to VB4) is set to a valuewhich will turn off the current control transistor 126 or 127, and oneof the currents I1 and I2 is cut off by such selection of the biasvoltage, the voltage VT1 or VT2 may be output as output voltage Vout.The reason is that, since the differential pair, the current of whichhas been cut off, becomes inoperative, the voltage output to the otherdifferential pair is output by the voltage follower operation.

Thus, the differential amplifier of FIG. 1 is able to output a voltageequal to any one of the two input voltages, or a voltage of any one ofplural voltage levels, interiorly dividing the two input voltages, bychanging the current ratio of the currents I1 and I2 flowing through thefirst differential pair (101, 102) and the second differential pair(103, 104) under control by the group of switches of the current controlcircuit 7.

The operation of the differential amplifier of the second embodiment ofthe present invention, described with reference to FIG. 2, will now bedescribed. The differential amplifier of the second embodiment of thepresent invention is able to selectively supply the two supply voltagesV1 and V2 (V1<V2), inclusive of overlapping values, by the input controlcircuit 8. In FIG. 2, the current ratio of the currents I1 and I2,supplied to the differential pair (101, 102) and to the seconddifferential pair (103, 104), is set to a constant value.

In case the same voltage is supplied to the terminals T1 and T2, by theinput control circuit 8, that is, in case one of the voltages V1 and V2is supplied in common to the terminals T1 and T2 (VT1=VT2), theoperation of the differential amplifier of FIG. 2 is equal to that of aroutine voltage follower, such that the output voltage Vout becomesequal to the voltage input common to the terminals T1 an T2.

The case in which the different voltages are supplied to the terminalsT1 and T2, by the input control circuit 8, that is, in case one of thevoltages V1 and V2 is supplied to the terminal T1 and the other issupplied to the terminal T2, will now be described. It is noted that, incase the current ratio of the currents I1 and I2 is such that I1>I2 orI1<I2, the output voltage Vout of the differential amplifier of FIG. 2is a voltage which interiorly divides the voltages VT1 and VT2 of theterminals T1 and T2 with a preset interior division ratio X:Y, excludingX=Y, as described with reference to FIG. 4.

If, in the case of the differential amplifier, where the current ratioof the currents I1 and I2 is fixed, there lacks the input controlcircuit 8, and the two voltages, supplied to the terminals T1 and T2,are fixed, the output voltage Vout is a sole level voltage whichinteriorly divides the voltages VT1 and VT2 of the terminals T1 and T2with a preset interior division ratio X:Y,

However, with the differential amplifier of FIG. 2, the voltagessupplied to the terminals T1 and T2 can be interchanged by the inputcontrol circuit 8. Hence, the output voltage Vout is able to assume alevel voltage corresponding to interior division X:Y or a level voltagecorresponding to interior division Y:X of the two voltages supplied tothe terminals T1 and T2.

That is, with the differential amplifier of FIG. 2, plural levels ofvoltages with different interior division ratios may be output byinterchanging voltages supplied to the terminals T1 and T2.

A modification of the differential amplifier of FIG. 2 will now bedescribed. The differential amplifier of FIG. 2 may be configured sothat the current ratio of the currents I1 and I2 is equal to unity(I1=I2) and so that the transistors of the differential pairs (101, 102)and (103, 104) are of different characteristics. The operation for thiscase will now be described with reference to FIG. 5.

FIG. 5 shows the operation of a case in which, in the two differentialpairs (101, 102) and (103, 104) of the differential amplifier of FIG. 2,the W/L ratio (ratio of the channel width W to the channel length L) ofthe differential pair (101, 102) is set to twice as large as that of thedifferential pair (103, 104).

FIG. 5 shows a case where the voltage V1 (VT1=V1) and the voltage V2(VT2=V2), where V1<V2, are selectively output to the terminals T1 andT2, respectively.

FIG. 5 shows, similarly to FIG. 3, the relationship between thedrain-to-source current Ids (ordinate) and the voltage V against thepower supply VSS (abscissa), and a characteristic curve 1 corresponds tothe transistors 101 and 102, while a characteristic curve 2 correspondsto the transistors 103 and 104, with the transistors of 101, 102 of thedifferential pair having operating points a and b on the characteristiccurve 1 and with the transistors of 103, 104 of the other differentialpair having operating points c and d on the characteristic curve 2.

It should be noted that, since the differential pairs (101, 102) and(103, 104) are formed to have different W/L ratios, the twocharacteristic curves are shifted from each other along the abscissa,with the slopes thereof being different from each other.

When the current ratio of the currents I1 and I2 is such that I1=I2, therelationship of the currents in the transistors 101 to 104 in thedifferential amplifier of FIG. 2 is similar to that shown in FIG. 3 andthe above equations (1) to (6) are valid.

The operating points a, b, c and d at this time are at the points ofintersection of the characteristic curves 1 and 2 and straight linesIds=Ia=Id and Ids=Ib=Ic. On the other hand, since the operating pointsb, d of the transistors 102 and 104 have V=Vout on the abscissa V ofFIG. 5 in common, the operating points are as shown in FIG. 5.

It is seen from FIG. 5 that the slope of the characteristic curve 1 istwice that of the characteristic curve 2. The output voltage Vout atthis time is such a voltage interiorly dividing the voltages VT1 and VT2at the terminals T1 and T2 by a ratio of 1:2. It is noted that thisinterior dividing ratio of the voltages VT1 and VT2 depends on the sizeratio of the two differential pairs (101, 102), (103, 104), so that, byoptimally setting the size ratio, it is possible to output a voltageinteriorly dividing by a desired interior division ratio.

Thus, when a voltage V1 (VT1=V1) and a voltage V2 (VT2=V2) areselectively supplied to the terminals T1 and T2, by the input controlcircuit 8, respectively, the output voltage Vout is such a voltage whichinteriorly divides the voltages V1 and V2 by 1:2, as shown in FIG. 5. Onthe other hand, in case a voltage V2 (VT1=V2) and a voltage V1 (VT2=V1)are selectively supplied to the terminals T1 and T2 by the input controlcircuit 8, respectively, the output voltage Vout is such a voltage whichinteriorly divides the voltages V1 and V2 by 2:1. Hence, thedifferential amplifier of FIG. 2 interchanges the voltages output to theterminals T1 and T2 by the input control circuit 8 to output voltages ofdifferent levels with the different interior division ratio.

A third embodiment of the present invention will now be described. FIG.6 is a diagram illustrating the configuration of a digital-to-analogconverter, abbreviated to ‘DAC’, according to a third embodiment of thepresent invention. Referring to FIG. 6, in the DAC of the thirdembodiment of the present invention, the amplifier 200 of the DAC andthe third group of the switches SW1 to SW3 of FIG. 18 are replaced by adifferential amplifier 300 employing the differential amplifier of thefirst embodiment of the present invention shown in FIG. 1.

In the DAC of FIG. 6, the means for selecting the two voltages, outputto the terminals T1 and T2 of the differential amplifier 300, is thesame as that used in FIG. 18. That is, the means for selecting the twovoltages is made up by a resistor string, outputting (n+1) analogvoltages V₁, V₂, . . . , and V_(n+1) from respective tap voltages, afirst set of switches, constituted by n switches S1 a to Sna, forselecting one voltage V_(K) from the respective taps, where K is aninteger from 1 to n, and a second set of switches, constituted by nswitches of from S1 b to Snb, for selecting a voltage V_(K+1) of a levelneighboring to the voltage V_(k). The means for selecting the twovoltages is able to turn on the K'th switches (Ska and Skb) of the firstand second groups of switches, by the output of an MSB subword decoder,which is based on the upper bit signals of the input data, to select thevoltage of the neighboring taps as V_(K) (=VT1) and V_(K+1) (=VT2).

The DAC according to the third embodiment of the present inventiondiffers from the DAC shown in FIG. 18 in that, in the present thirdembodiment, the respective switches of the current control circuit 7 arecontrolled by an output of an LSB subword decoder which is based onlower bit signals of input data. That is, in the third embodiment of thepresent invention, voltages of plural levels interiorly dividing thevoltages V_(K) and V_(K+1) output to the terminals T1 and T2 may beoutput. Since the number of the differential pairs is not increased,even though the number of voltage levels interiorly dividing thevoltages V_(K) and V_(K+1) is increased, marked area saving may beachieved as compared to the configuration of FIG. 18. If, in the presentthird embodiment, the number of voltage levels interiorly dividing thevoltages V_(K) and V_(K+1) is increased, the number of bias voltages tobe supplied to the control terminals of the current control transistors126 and 127, or that of the switches for switching the bias voltages isincreased, however, the bias voltages may be shared by pluraldifferential amplifiers 300. Since the switch may be formed by atransistor of the minimum size, there is only small adverse effect onincreasing the area. On the other hand, if the number of voltage levels,interiorly dividing the voltages V1 and V2, selected by the output ofthe MSB subword decoder, is increased, the number of the groups of thedifferential pairs, the number of the current sources driving them andthe number of the groups of the third switches for selectivelyoutputting the voltages V1 and V2 to one ends of the differential pairsare increased. Since the variations in the properties of the transistorsforming the differential pair and the current source directly affect theaccuracy of the linear output, these transistors need to be designed toa more or less large size, thus leading to an increased area.

If the principle of the DAC of FIG. 6 is applied to the gray scalevoltage generating circuit 986, decoder 987 and to the buffer circuit988 of FIG. 17, the number of gray level voltages, output from the grayscale voltage generating circuit 986, may be appreciably reduced to onehalf of the number of gray scale voltages of FIG. 16, or less. Inaddition, the number of the devices of the decoder 987, selecting thegray scale voltages, may be reduced, thus appreciably reducing thecircuit scale.

FIG. 7 is a diagram illustrating the configuration of adigital-to-analog converter (DAC) according to a fourth embodiment ofthe present invention. In the DAC shown in FIG. 18 or in FIG. 6, twovoltages (V_(K), V_(K+1)) of neighboring taps of n voltages, generatedby a resistor string, are selected and supplied to the differentialamplifier. In the fourth embodiment of the present invention, shown inFIG. 7, the number of tap voltages may be decreased with respect to thenumber of output voltage levels, by selecting not only the twoneighboring tap voltages but also any optional tap voltages, such as twotap voltages with one tap in-between or with two taps in-between.

Referring to FIG. 7, the means for selecting two voltages, output to theterminals T1 and T2 of a differential amplifier 400, includes a resistorstring for outputting n analog voltages V1, V2, . . . , and Vn fromrespective taps, a first group of n switches S1 a to Sna for selecting asole voltage Vs from the taps, where S denotes an integer from 1 to n,and a second group of n switches S1 b to Snb for selecting a solevoltage V1 from the taps, where J denotes an integer from 1 to n. Anumber S switch Ssa and a number J switch Sjb are turned on by an outputof a decoder which is based on the totality of bit signals of the inputdata (MSB+LSB) to select a combination of optional tap voltages,including overlapping values (Vs and V1) as the voltages (VT1, VT2) ofthe terminals T1 and T2. Meanwhile, the DAC of FIG. 7 may be regarded asan extension by n of the number of voltage supply terminals of the inputcontrol circuit 8 of FIG. 2.

In the DAC of FIG. 7, a differential amplifier 400 is desirably such adifferential amplifier capable of maintaining a preset interior divisionratio to provide for a high precision output even for a wide voltagedifference area of two voltages output to terminals T1 and T2. Forexample, the differential amplifier capable of outputting anintermediate voltage of two input voltages, described with reference toFIG. 3, is among these desirable types of the differential amplifiers.That is, such a differential amplifier, shown in FIG. 2, in which twodifferential pairs (101, 102) and (103, 104) are formed by transistorsof the same characteristics, and in which the current ratio of thecurrents I1, I2 flowing through the two differential pairs (101, 102)and (103, 104) is equal to unity (I1=I2), may be used.

The operation of the differential amplifier 400 is the same as thatdescribed with reference to FIG. 3. That is, the output voltage Vout isa voltage which interiorly divides the voltages VT1 and VT2 at theterminals T1 and T2 by a ratio of 1:1. This operation is establishedwithout regard to the potential difference of the voltages VS (=VTI) andVJ (=VT2), supplied to the terminals T1 and T2 in FIG. 7, and alsowithout regard to the profile of the characteristic curve if only thetransistors of the two differential pairs are of the samecharacteristics. Meanwhile, if, in case the transistors of thedifferential pairs exhibit e.g. the channel-length-modulation effect orthe like, the voltage difference between the two voltages VS and VJ isincreased excessively, there are occasions where the output voltage Voutis deviated gradually from the intermediate voltage. In general, theoperation is established over a relatively wide range.

A specified embodiment in which the number of tap voltages may bedecreased as compared to the number of output voltages by selection ofoptional combinatorial voltages (Vs, Vj) of the first and second groupsof switches of FIG. 7 will now be described. In the following specifiedembodiment, such a case is described in which a linear output with equalintervals between output voltage levels is to be implemented.

FIG. 8 shows an example of a method for selecting the two input voltagesto be output to the terminals T1 and T2 of the differential amplifier400 of FIG. 7. FIG. 8 shows, in a tabulated form, nine equally spacedapart level voltages, four voltages A to D, different from one another,output from respective taps of the resistor string of FIG. 7, and thecombination of two voltages, entered to the terminals T1 and T2.Meanwhile, it does not matter which one of the combinations of the twovoltages is output to which one of the terminals T1 and T2.

In FIG. 8, the number of voltages supplied to the terminals T1 and T2 isonly four against the nine level output voltages, that is, less thanone-half of the number of the output voltage levels. However, the twovoltages may be combined so that, when the voltage A is selectivelysupplied to one of the two terminals (T1, T2), any of voltages A, B, Cand D, that is, four voltages, may be supplied to the other terminal.Thus, there are a sum total of ten combinations of two out of fourvoltages, such that a 9-level linear output is possible.

The four input voltages A to D are set to first, third, seventh andninth voltage levels, against the nine-level output voltages.

As for the combination of two voltages, output to the terminals (T1,T2), accomplishing the fifth level output, the combination of thevoltages B and C or the combination of the voltages A and D may be used.

In FIG. 8, the first to eighth levels, out of the nine-level outputvoltages, may be associated with (0, 0, 0) to (1, 1, 1) for three-bitdigital data.

FIG. 9 shows another specified example of the method for selecting twovoltages output to the terminals T1 and T2 of the differential amplifier400 of FIG. 7.

FIG. 9 shows, in a tabulated form, 17 equally spaced apart levelvoltages, six voltages A to F, different from one another, output fromrespective taps of the resistor string of FIG. 7, and the combination oftwo voltages, output to the terminals T1 and T2. Meanwhile, it does notmatter which one of the combinations of the two voltages is output towhich one of the terminals T1 and T2.

In FIG. 9, the number of voltages supplied to the terminals T1 and T2 isonly six against the 17 level output voltages, that is, approximatelyone-third of the number of the output voltage levels. However, there are21 combinations of two out of six voltages, whereby 17-level linearoutputs are feasible. The four voltages A to F are set in the first,third, seventh, eleventh, fifteenth and seventeenth voltage levels,respectively. There are plural combinations of the bi-level inputsoutputting the seventh, ninth and eleventh levels. In FIG. 9, the firstto sixteenth levels, out of the seventeen level output voltages, may beassociated with (0, 0, 0, 0) to (1, 1, 1, 1) for four-bit digital data.

In FIGS. 8 and 9, typical examples for reducing the number of voltagessupplied from the taps of the resistor string of FIG. 7 to thedifferential amplifier capable of outputting an intermediate voltage oftwo voltages supplied to the terminals T1 and T2, are shown for 9-leveland 17-level linear outputs. However, the present invention may beapplied to linear outputs of larger numbers of multi-levels.

According to the present invention, if the number of linear outputvoltage levels is m, m being a multiple of 8 plus 1, the number ofvoltages supplied from the respective taps of the resistor string ofFIG. 7, that is, the number of voltages supplied to the terminals T1 andT2, may be reduced to [{(m−1)/4}+2].

In this case, the voltages supplied from the respective taps of theresistor string of FIG. 7 are set to the first, (3+4k)'th and m'thlevels, against m output levels, k is set to an integer not less than 0,and the (3+4k)'th level is set to include the (m−2)'th level.

FIG. 8 shows a case where m=9 and k=0, 1 and FIG. 9 shows a case wherem=17 and k=0, 1, 2, 3.

As described above, in the DAC of FIG. 7, the differential amplifier 400is able to output only voltages having a fixed interior division ratiorelative to the two input voltages T1 and T2. However, the DAC may beadapted to output a large number equal to twice or more of the nvoltages output from the taps of the resistor string, by selecting thecombination voltages by the first and second groups of switchesresponsive to the input data.

By applying the DAC of the embodiment of the present invention to thegray scale voltage generating circuit 986, decoder 987 and to the buffercircuit 988 of FIG. 17, the number of gray level voltages, output fromthe gray scale voltage generating circuit 986, may be appreciablyreduced to one half of the number of gray level voltages of FIG. 16, orless. In addition, the number of the devices of the decoder 984,selecting the gray level voltages, may be reduced, thereby reducing thecircuit scale appreciably.

Although the present embodiments of the differential amplifiers anddigital-to-analog converters according to the present invention aredescribed in the foregoing, the configurations of FIGS. 1 to 7 may bereplaced by transistors formed without back gates on an insulatingsubstrate, formed of glass or plastics. In such case, the operation andeffect similar to those described above may similarly be achieved.

With a data driver, having a differential amplifier or adigital-to-analog converter according to the present invention, it ispossible to reduce the decoder area to enable cost reduction. If thedata driver is used as a data driver of FIG. 15, it becomes possible toachieve cost reduction of the liquid crystal display apparatus.

Meanwhile, in the liquid crystal display apparatus of FIG. 15, the datadriver 980 may be connected distinctly as a silicon LSI and connected tothe display 960. Or, a circuit may be formed using e.g. polysilicon TFT(thin-film transistor) as an insulating substrate, such as a glasssubstrate, for forming the data driver as one with the display 960. Inparticular, if the data driver is formed as one with the display, thedata driver may be reduced in area, so that it is possible to reduce theframe portion (that is, the width between the outer rim of the display960 and the outer rim of the substrate).

By applying the differential amplifier of the present invention to anydata driver of the display apparatus, including the display apparatus ofthe alternative configuration, it is possible to save the cost and tonarrow the frame portion. For example, the differential amplifier of thepresent invention may, of course, be applied to a display apparatusoutputting and demonstrating multi-level voltage signals on a data line,similarly to the liquid crystal display apparatus, such as an organic ELdisplay of the active matrix driving system.

Referring to the drawings, the results of simulation carried out inorder to verify the operation and favorable effect of the differentialamplifiers of the above mentioned embodiments of the present invention,will now be described.

FIG. 10 is a diagram illustrating a simulation circuit for adifferential amplifier for verifying the results of the first embodimentof the present invention. FIG. 10 also shows a specified example of thefirst embodiment shown in FIG. 1. An amplifier stage 6 comprises aP-channel transistor 109, which is for charging the output terminal 3,and a current source 110 which is connected between the output terminal3 and the power supply VSS to discharge the output terminal 3. In otherrespects, the configuration is similar to that shown in FIG. 1.

A transistor 109 is connected between the high potential power supplyVDD and the output terminal 3 and has a gate connected to an outputterminal 4 (drain of the transistor 111) of the current mirror circuit(111, 112). The current source 110 is connected between the lowpotential power supply VSS and the output terminal 3. Although not shownin FIG. 10, a phase compensation capacitor, not shown, is provided asnecessary across the transistor 109 and the output terminal 3.

FIG. 11 shows input/output characteristics (simulation) when thedifferential pair (101, 102) and the second differential pair (103, 104)are formed by transistors of the same characteristics, the current ratioof the currents I1, I2 is equal to unity (I1=I2) and the operation ofFIG. 3 of outputting an intermediate voltage of the voltages VT1 and VT2at the terminals T1 and T2 is carried out.

FIG. 11 shows output characteristics of the output voltage Vout when thevoltage VT1, out of the two voltages VT1 and VT2 at the terminals T1 andT2 of the differential amplifier, is constant and when the voltage VT2is varied within a range of ±0.5V relative to VT1.

At this time, the output voltage Vout becomes a voltage bisecting thetwo voltages (VT1, VT2). An expected output value is shown by a brokenline in FIG. 11.

It is seen from FIG. 11 that the output voltage Vout shows goodcoincidence with an expected output value within a range of ±0.5V, with5V as center, and that a voltage intermediate between the two inputvoltages may be output to a high accuracy over a wide voltage range.

The above results are valid for a DAC of the fourth embodiment of thepresent invention shown in FIG. 7. If, in the differential amplifier 400shown in FIG. 7, the two differential pairs (101, 102) and (103, 104)are formed by transistors of the same characteristics and the currentsI1, I2 flowing through the respective differential pairs are equal(I1=I2), the intermediate voltage of the voltages VT1 and VT2 of theterminals T1 and T2 may be output to high accuracy over a wide voltagerange. Hence, the meritorious effect, described with reference to FIGS.8 and 9, may be achieved, such that it becomes possible to reduceappreciably the number of tap voltages as compared to the number ofoutput voltages.

FIG. 12 depicts input/output characteristics resulting from thesimulation in which the two differential pairs (101, 102) and (103, 104)of the differential amplifier of FIG. 10 are constituted by transistorsof the same characteristics and the current ratio of the respectivecurrents I1 and I2 flowing through the differential pairs is changedunder control by the current control circuit 7.

FIG. 12 shows input/output characteristics resulting from the simulationin which the current ratio of the currents I1 and I2 is switched to 2:1and to 1:2. When the current ratio of the currents I1 and I2 is switchedto 2:1 and to 1:2, the output voltage Vout becomes Vo2 and Vo3,respectively. Also, the expected output values in case the voltages VT1and VT2 at the terminals T1 and T2 are interiorly divided by the ratiosof 1:2 and 2:1 are denoted by broken lines labeled expected output valueVo2 and expected output value Vo3, respectively.

It may be confirmed from FIG. 12 that the interior division ratio of thevoltages VT1 and VT2 at the terminals T1 and T2 approximatelycorresponds to a reciprocal of the current ratio of the currents I1 andI2, and that the output voltage Vout approximately coincides with theexpected output values (expected output value Vo2 and expected outputvalue Vo3) within a range of ±0.1V. Meanwhile, the output voltage Voutis progressively deviated from the expected output value for the voltagerange in which the output voltage exceeds ±0.1V.

FIG. 13 shows the input/output characteristics resulting from thesimulation in which the two differential pairs (101, 102) and (103, 104)of the differential amplifier of FIG. 10 are formed by transistors ofthe same characteristics and the current ratio of the currents I1 and I2flowing through the respective differential pairs is changed to 1:3, 1:1and to 3:1 under control by the current control circuit 7.

In FIG. 13, in case the current ratio of the currents I1 and I2 ischanged to 3:1, 1:1 and to 1:3, the output voltage Vout becomes Vo2, Vo3and Vo4, respectively. Also, the expected output values, when thevoltages at the terminals T1 and T2 (VT1, VT2) are interiorly divided by1:3, 1:1 and to 3:1, are denoted by broken lines labeled expected outputvalue Vo2, expected output value Vo3 and expected output value Vo4,respectively.

It may be confirmed from FIG. 13 that the interior division ratio of thevoltages VT1 and VT2 at the terminals T1 and T2 substantiallycorresponds to the reciprocal of the current ratio of the currents I1and I2, such that the output voltage Vout substantially coincides withthe output expected values (indicated by broken lines) within a range of±0.1V. As for the voltage range exceeding ±0.1V, the output voltages Vo2and Vo4 are progressively deviated from the expected output value Vo2and from the expected output value Vo4, respectively, however, theoutput voltage Vo3, with the current ratio of the currents I1 and I2equal to 1:1, is coincident fairly well to the expected output valueVo3, over a wide voltage range, as in FIG. 11.

In FIGS. 12 and 13, there are shown cases where the ratio of interiordivision of the voltages VT1 and VT2 by the output voltage Voutsubstantially coincides with the reciprocal of the current ratio of thecurrents I1 and I2. This relationship may need the proper settings ofthe transistor characteristics and the current ratio of the currents I1and I2.

From FIGS. 12 and 13, it has been shown that, with the differentialamplifier of the first embodiment of the present invention, it ispossible to output plural voltage levels, interiorly dividing the twovoltages, supplied to the terminals T1 and T2. It has also been shownthat plural voltage levels, including the same level as the inputvoltage level, can be linearly output. The DAC of the third embodimentof the present invention, employing the differential amplifier of thefirst embodiment of the present invention, may also yield comparableresults.

FIG. 14 shows input/output characteristics resulting from the simulationin which an amplifier stage 6 similar to one shown in FIG. 10 is used inthe differential amplifier of the second embodiment of the presentinvention shown in FIG. 2. Specifically, FIG. 14 shows input/outputcharacteristics under the condition that the transistor size W/L of thedifferential pair (101, 102) is set to a value twice that of thedifferential pair (103, 104), the current ratio of the currents I1 andI2 flowing through the differential pairs is set to unity (I1=I2) andthat the input to the terminals T1 and T2 of the two supply voltages V1and V2 is controlled by the input control circuit 8.

If, in FIG. 14, the voltages output to the terminals (T1, T2) isswitched from (V1, V2) to (V2, V1), the output voltage Vout becomes Vo2and Vo3, respectively. The expected output values, interiorly dividingthe voltages (V1, V2) by 1:2 and 2:1 are denoted by broken lines labeledexpected output value Vo2 and expected output value Vo3, respectively.

It may be conformed from FIG. 14 that the output voltage Vout roughlycoincides with the expected output value (broken line) within a range of±0.1V. Meanwhile, in a voltage range exceeding ±0.1 V, the outputvoltage Vout is gently deviated from the expected output value. It hasbeen shown from FIG. 14 that the differential amplifier of the secondembodiment of the present invention is able to output plural voltagelevels, interiorly dividing the two voltages, input to the terminals T1and T2, as well as to output plural linear voltage levels including thesame level as the input two voltage levels.

As may be seen from the above four results of simulation, it is possiblewith the differential amplifier of the present embodiment to output avoltage which divides the two voltages output to the terminals T1 and T2into plural voltages and, in particular, for a voltage range with arelatively small voltage difference, for example, the voltage differenceon the order of 0.1V, it is possible to output plural divided voltagesto a high accuracy.

FIG. 20 shows an arrangement of a differential amplifier according to afifth embodiment of the present invention. In the embodiments shown inFIGS. 1 and 6, the differential amplifier is provided with the twodifferential pairs (101, 102) and (103, 104) having the sameconductivity type. However, the differential amplifier is provided withtwo sets of the two differential pairs, the respective set beingdifferent in conductivity type each other. The differential amplifier ofFIG. 20 is of the configuration in which there are provided twodifferential pairs of the same conductivity type and two otherdifferential pairs of the same conductivity type different from that ofthe first-stated differential pairs, and is superior to the differentialamplifier of, for example FIG. 1, in enlarging the dynamic range and inimproving linearity. Referring to FIG. 20, the differential amplifierincludes an n-channel differential pair (101, 102), which is driven by acurrent source 126 which is connected to the low potential power supplyVSS, an n-channel differential pair (103, 104), which is driven by acurrent source 127, which is connected to the low potential power supplyVSS, a current mirror circuit (p-channel transistors 111 and 112), whichis connected between output pairs of the two n-channel differentialpairs and a high potential power supply VDD and composes a common activeload for the output pairs of the two n-channel differential pairs, andan amplifier circuit 6, which is supplied with an output signal of thecurrent mirror circuit to output a voltage at an output terminal 3. InFIG. 20, the groups of switches SW1 a, SW1 b to SW4 a, SW4 b of FIG. 1are not shown for convenience. However, the current control circuit 7for controlling the current sources 126 and 127, adapted for providingthe currents I1 and I2 to the two N-channel differential pairs (101,102), (103, 104), is of the same configuration as that shown in FIG. 1.

The differential amplifier also includes a p-channel differential pair(201, 202) which is driven by a current source 226 which is connected tothe high potential power supply VDD, a p-channel differential pair (203,204) which is driven by a current source 227 which is connected to thehigh potential power supply VDD, a current mirror circuit (n-channeltransistors 211, 212) which is connected between the output pairs of thetwo p-channel differential pairs and the low potential power supply VSS,and composes a common active load for the output pairs of the twop-channel differential pairs, and an amplifier circuit 16, which issupplied with an output signal of the current mirror circuit to output avoltage at the output terminal 3. A current control circuit 17 forcontrolling the current sources (226, 227), which is adapted forsupplying the currents I11 and I12 to the two P-channel differentialpairs (201, 202), (203, 204), is similar in configuration to the currentcontrol circuit 7, although it is not shown for simplifying thedrawings. As for the input terminals (gate terminals) of the respectivedifferential pairs, the gates of the transistors 101, 201 are connectedin common to the input terminal T1, the gates of the transistors 103 and203 are connected in common to the input terminal T2, and the gates ofthe transistors 102 and 104, and the gates of the transistors 202 and204 are connected in common to the output terminal 3. The terminalvoltages at the terminals T1 and T2 are designated as VT1 and VT2,respectively.

With the differential amplifier of FIG. 20, an output voltage,interiorly dividing the two voltages (VT1, VT2) at an optional ratio,may similarly be taken out by the current control circuits 7 and 17. Inthe embodiments of FIGS. 2 and 7, the differential amplifier is providedwith two sets of the two differential pairs, and the two transistors ofeach set of the differential pairs may be of the different conductivitytypes. The differential amplifier of FIG. 20 may, of course, be designedso that the current sources 126 and 127, and the current sources 226 and227 are set to respective preset current values, the input controlcircuit 8 is provided to the terminals T1 and T2 or that there isfurther provided a resistor string similar to that shown in FIG. 7 and aset of switches S1 a to Snb of FIG. 7.

In the embodiments shown in FIGS. 1, 2, 6 and 7, the signal at one nodeof a pair of connection nodes between the output pairs of the twodifferential pairs (101, 102) and (103, 104) and the load circuit (111,112) is supplied to the amplifier stage 6. Alternatively, it is alsopossible to construct the amplifier stage 6 by a differential amplifierstage and a pair of connection nodes between the output pairs of the twodifferential pairs (101, 102) and (103, 104) and the load circuit (111,112) may differentially be connected to the inputs of an input pair ofthe amplifier stage. In the embodiments shown in FIGS. 1, 2, 6 and 7,the load circuits 111, 112 are used in common by the two differentialpairs (101, 102) and (103, 104). However, distinct load circuits may, ofcourse, be provided for these differential pairs.

Although the present invention has so far been described with referenceto specified embodiments thereof, the present invention is not limitedto these embodiments and a variety of changes or corrections that mayreadily occur to those skilled in the art may naturally be attemptedwithout departing from the scope of the invention as defined in theclaims.

The differential amplifier, described in the above embodiments, isformed by MOS transistors. In a driving circuit for a liquid crystaldisplay apparatus, a MOS transistor of polycrystalline silicon (TFT) maybe used. Although the above embodiments are directed to the integratedcircuit, discrete device structures may, of course, be used within thescope of the present invention.

It should be noted that other objects, features and aspects of thepresent invention will become apparent in the entire disclosure and thatmodifications may be done without departing the gist and scope of thepresent invention as disclosed herein and claimed as appended herewith.

Also it should be noted that any combination of the disclosed and/orclaimed elements, matters and/or items may fall under the modificationsaforementioned.

1. A differential amplifier comprising: first and second inputterminals; an output terminal; first and second differential pairs;first and second current sources for supplying currents to said firstand second differential pairs, respectively; said first differentialpair having first and second inputs constituting an input pair thereofconnected to said first input terminal and said output terminal,respectively; said second differential pair having first and secondinputs constituting an input pair thereof connected to said second inputterminal and said output terminal, respectively; a load circuitconnected to output pairs of said first and second differential pairs,for outputting a signal obtained on combining outputs of said first andsecond differential pairs from at least one of respective connectionnode between the output pairs of said first and second differentialpairs and said load circuit; an amplifier stage for receiving at leastone signal at a connection node between said output pairs of said firstand second differential pairs and said load circuit and outputting avoltage at said output terminal; and a current control circuit forcontrolling said first and second current sources to control the ratioof currents supplied to said first and second differential pairs,respectively.
 2. The differential amplifier according to claim 1,wherein said load circuit is composed by a current mirror circuit. 3.The differential amplifier according to claim 1, further comprising:first and second voltage supply terminals for receiving preset voltages;and an input control circuit for controlling the connection between saidfirst and second voltage supply terminals and said first and secondinput terminals.
 4. The differential amplifier according to claim 1,wherein said first and second current sources each including atransistor; said current control circuit for selecting a bias voltage,out of a plurality of bias voltages received, based on an input controlsignal, and supplying the selected bias voltage to control terminals ofsaid transistors constituting said first and second current sources tocontrol the ratio of the currents to be supplied to said first andsecond differential pairs, respectively.
 5. The differential amplifieraccording to claim 1, wherein said current control circuit controls saidfirst and second current sources so that the sum of the currentssupplied to said first and second differential pairs is kept constant.6. The differential amplifier according to claim 1, wherein thetransistor pairs of said first and second differential pairs are of thesame size; and the ratio of the currents through said first and seconddifferential pairs is adjusted; a voltage corresponding to one of levelswhich divide the potential difference between said first and secondinput terminals by three being output from said output terminal.
 7. Thedifferential amplifier according to claim 1, wherein the currents ofsaid first and second current sources differ from each other; and firstand second voltages are applied to said first and second inputterminals, respectively; wherein a first voltage level, corresponding tosaid first voltage, is output from said output terminal, underinterruption of the current of said second current source; a secondvoltage level, corresponding to one of voltage levels which divide thepotential difference between said first and second voltages by three, isoutput from said output terminal, with the current of said first currentsource being larger than the current of said second current source; athird voltage level, corresponding to the other of voltage levels whichdivide the potential difference between said first and second voltagesby three, is output from said output terminal, with the current of saidsecond current source being larger than the current of said firstcurrent source; and a fourth voltage level, corresponding to said secondvoltage, is output from said output terminal, under interruption of thecurrent of said first current source.
 8. The differential amplifieraccording to claim 1, further comprising: third and fourth differentialpairs, the conductivity type of which is different from that of saidfirst and second differential pairs, said third and fourth differentialpairs having first inputs constituting respective input pairs thereofconnected to said first and second input terminals, respectively andhaving second inputs constituting the respective input pairs thereofconnected to said output terminal; third and fourth current sources forsupplying the current to said third and fourth differential pairs,respectively; a further load circuit connected to output pairs of saidthird and fourth differential pairs; at least one output of an outputpair of said third differential pair and one output of an output pair ofsaid fourth differential pair being connected in common; a furtheramplifier stage having an input terminal connected to a commonconnection node between one output of an output pair of the thirddifferential pair and one output of an output pair of the fourthdifferential pair and having an output terminal connected to said outputterminal; and a further current control circuit for controlling saidthird and fourth current sources to control the ratio of currentssupplied to said third and fourth differential pairs.
 9. A differentialamplifier comprising: first and second input terminals; an outputterminal; first and second differential pairs having at least twotransistor characteristics different from each other; and first andsecond current sources for supplying the currents to said first andsecond differential pairs, respectively; said first differential pairhaving first and second inputs constituting an input pair thereofconnected to said first input terminal and said output terminal,respectively; said second differential pair having first and secondinputs constituting an input pair thereof connected to said second inputterminal and said output terminal; a load circuit connected to outputpairs of said first and second differential pairs, for outputting asignal obtained on combining outputs of said first and seconddifferential pairs from at least one of a pair of connection nodesbetween said output pairs of said first and second differential pairsand said load circuit; and an amplifier stage for receiving at least onesignal at a connection node between said output pairs of said first andsecond differential pairs and said load circuit and outputting a voltageto said output terminal.
 10. The differential amplifier according toclaim 9, further comprising: first and second voltage supply terminalsfor receiving preset voltages, respectively; and an input controlcircuit for controlling the connection of said first and second voltagesupply terminals and said first and second input terminals.
 11. Thedifferential amplifier according to claim 9, wherein said first andsecond current sources have the same current values; the size ratio ofthe transistor pairs of said first and second differential pairs beingadjusted; a voltage corresponding to one of voltage levels which dividethe potential difference between said first and second input terminalsbeing output from said output terminal by three.
 12. The differentialamplifier according to claim 9, wherein the size ratio of the transistorpairs of said first and second differential pairs is adjusted; and saidfirst and second current sources have the same current values; wherein afirst voltage level, corresponding to said first voltage is output fromsaid output terminal, with said first and second input terminals bothbeing at a first voltage; a second voltage level corresponding to one ofvoltage levels which divide the potential difference between said firstand second voltages by three, is output from said output terminal, withsaid first and second input terminals being at first and secondvoltages, respectively; a third voltage level corresponding to one ofvoltage levels which divide the potential difference between said firstand second voltages by three, is output from said output terminal, withsaid first and second input terminals being at second and firstvoltages, respectively; and a fourth voltage level corresponding to saidsecond level is output from said output terminal, with said first andsecond input terminals both being at said second voltage.
 13. Adifferential amplifier comprising: first and second input terminals; anoutput terminal; first and second differential pairs; first and secondcurrent sources for supplying the current to said first and seconddifferential pairs; said first differential pair having first and secondinputs constituting an input pair thereof connected to said first inputterminal and said output terminal, respectively; said seconddifferential pair having first and second inputs constituting an inputpair thereof connected to said second input terminal and said outputterminal, respectively; a load circuit connected to output pairs of saidfirst and second differential pairs, for outputting a signal obtained oncombining outputs of said first and second differential pairs from atleast one of a pair of connection nodes between said output pairs ofsaid first and second differential pairs and said load circuit; anamplifier stage for receiving at least one signal at connection nodes ofsaid output pairs of said first and second differential pairs and saidload circuit and outputting a voltage to said output terminal; and aselection circuit for receiving a plurality of voltages different fromone another, selecting, from among said voltages different from oneanother, two voltages, including the same voltage, based on a selectioncontrol signal, and supplying the selected two voltages to said firstand second input terminals; one output voltage being output for acombination of said two voltages selected by said selection circuit. 14.The differential amplifier according to claim 13, wherein the number ofsaid voltages, different from one another, is [{(m−1)/4}+2], m being amultiple of 8 plus 1; said selection circuit selects two voltages,inclusive of the same values, from among said [{(m−1)/4}+2] voltages, tosupply the selected two voltages to said first and second terminals; andwherein the differential amplifier is able to output m voltage levels atsaid output terminal and wherein, for the first to m'th voltage levelsoutput, the voltages supplied as input voltages include first, (3+4×k)thand the m'th voltage levels, k being an integer of 0, 1, . . . ,(m−5)/4, are included.
 15. The differential amplifier according to claim1, wherein said amplifier stage includes a transistor connected betweena power supply and sand output terminal and adapted forcharging/discharging said output terminal.
 16. The differentialamplifier according to claim 8, wherein at least one of said amplifierstage and said further amplifier stage includes a transistor connectedbetween a power supply and sand output terminal and adapted forcharging/discharging said output terminal.
 17. A digital-to-analogconverter comprising: a resistor string connected between said firstvoltage supply terminal and a second voltage supply terminal andoutputting a plurality of voltage levels different from one another fromrespective taps thereof; and a switching circuit for selectivelyoutputting two voltages, based on input data, from among a plurality ofvoltage levels, different from one another, output from respective tapsof said resistor string; and a differential amplifier as defined inclaim 1, as an amplifier circuit supplied with the two voltages outputfrom said switch circuit and outputting a voltage relating to said inputdata.
 18. A digital-to-analog converter comprising: a resistor stringconnected between a first voltage supply terminal and a second voltagesupply terminal and outputting a plurality of voltage levels differentfrom one another from respective taps thereof; and a switching circuitfor selectively outputting two voltages, inclusive of the same voltages,based on input data, from among a plurality of voltage levels, differentfrom one another, output from respective taps of said resistor string;and a differential amplifier as defined in claim 9, as an amplifiercircuit supplied with the two voltages output from said switch circuitand outputting a voltage relating to said input data.
 19. A data driverfor a display apparatus including the differential amplifier as definedin claim 9, as a buffer circuit.
 20. A data driver for a displayapparatus comprising: a gray level voltage generating circuit generatinga plurality of voltage levels; a decoder outputting at least twovoltages selected from said plural voltages, based on input data; and adifferential amplifier as defined in claim 9, as an amplifier circuitsupplied with the voltage output from said decoder and outputting avoltage relating to said input data at an output terminal.